To enable subgroups:

• enable posting into context: Studio > Settings > General > Allow to post to context using Privacy field
• allow only members of parent groups to join subgroup: Studio > Settings > General > Only parent group(space, event, etc) members can join
• allow to post Groups into Groups: Studio > Groups > Allow posting groups in the following contexts (requires additional setup) and execute the following SQL query:

UPDATE `sys_objects_privacy` SET `spaces` = 'bx_events,bx_spaces,bx_groups' WHERE `object` = 'bx_groups_allow_view_to'